This invention relates in general to the field of wireless communication systems, and more particularly to spread spectrum CDMA communications with antenna arrays.
The communication infrastructure of future wireless services will involve high-speed networks, central base stations, and various nomadic mobile units of different complexity that must interoperate seamlessly. In addition to standard issues such as capability and affordability, a mobile wireless network also emphasizes survivability against fading and interference, system flexibility and robustness, and fast access. Innovative communication technologies are strategically important to the realization of high performance Personal Communications Services (PCS) systems (D. Goodman, "Trends in Cellular and Cordless Communications", IEEE Communications Magazine, June 1991).
In PCS and other wireless communication systems, a central base station communicates with a plurality of remote terminals. Frequency-division multiple access (FDMA) and Time-division multiple access (TDMA) are the traditional multiple access schemes to provide simultaneous services to a number of terminals. The basic idea behind FDMA and TDMA techniques is to slice the available resource into multiple frequency or time slots, respectively, so that multiple terminals can be accommodated without causing interference.
Contrasting these schemes which separate signals in frequency or time domains, Code-division multiple access (CDMA) allows multiple users to share a common frequency and time channel by using coded modulation. In addition to bandwidth efficiency and interference immunity, CDMA has shown real promise in wireless applications for its adaptability to dynamic traffic patterns in a mobile environment. Because of these intrinsic advantages, CDMA is seen as the generic next-generation signal access strategy for wireless communications. However, current commercial CDMA technologies, e.g., the IS-95 standard developed by Qualcomm, still have substantial practical problems, the most important being a stringent requirement for accurate and rapid control of a terminals' transmission power. Although the power control problem may be alleviated by the use of synchronous CDMA (S-CDMA) techniques, this introduces other problems in, e.g., synchronization. For more information, please refer to M. K. Simon et al., "Spread Spectrum Communications Handbook", McGraw-Hill, 1994; Bustamante et al., "Wireless Direct Sequence Spread Spectrum Digital Cellular Telephone System, U.S. Pat. No. 5,375,140, December 1994; Schilling, "Synchronous Spread-Specturm Communications System and Method", U.S. Pat. No. 5,420,896, May 1995.
It is well known that given a fixed amount of frequency allocation, there exists an upper limit on the number of channels available for reliable communications at a certain data rate. Therefore, the aforementioned schemes can only increase the system capacity and performance to a certain extent. To exceed this limit, additional resources need to be allocated. The most recent attempts to increase system capacity and performance have attempted to exploit spatial diversities. The new dimension, i.e., space, when properly exploited by the employment of multiple antennas, can in principle lead to a significant increase in the system capacity (S. Andersson et al., "An Adaptive Array for Mobile Communication Systems," IEEE Trans. on Veh. Tec., Vol. 4, No. 1, pp 230-236, 1991; J. Winters et al., "The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems, IEEE Trans. on Communications, Vol. 42, No. 2/3/4, pp 1740-1751, 1994.) Other potential benefits include lower power consumption, higher immunity against fading and interference, more efficient handoff, and better privacy. A wireless communications system that utilizes adaptive antenna arrays is hereafter referred to as a Smart Antenna System. Despite of its promises however, many practical problems exist in smart antenna applications. For many reasons and mostly due to limitations of the existing wireless protocols, it is generally difficult to integrate the state-of-the-art antenna array technologies into current systems.
Sectorization, i.e., partitioning a coverage area into sectors by the use of directive antennas, is one of the straightforward means of exploiting the spatial diversity for capacity and performance advances. There have been a significant number of studies and patents in this area including S. Hattori, et al., "Mobile Communication System," U.S. Pat. No. 4,955,082, January 1989; T. Shimizu, et al., "High Throughput Communication Method and System for a Digital Mobile Station When Crossing a Zone Boundary During a Session," U.S. Pat. No. 4,989,204, December 1989; V. Graziano, "Antenna Array for a Cellular RF Communications System," U.S. Pat. No. 4,128,740, 13/1977. The sectorization approaches, however simple, have fundamental difficulties in handling the everchanging traffic pattern. As a result, sectorization offers only a limited capacity increase at the expense of more handoffs and complicated administration.
To accommodate the time-varying nature of mobile communications, adaptive antenna array technologies have been investigated; see e.g., K. Yamamoto, "Space Diversity Communications System for Multi-Direction Time Division Multiplex Communications", U.S. Pat. No. 4,599,734, April. 1985; D. F. Bantz, "Diversity Transmission Strategy in Mobile/Indoor Cellular Radio Communications", U.S. Pat. No. 5,507,035, April 1993; C. Wheatley, "Antenna System for Multipath Diversity in an Indoor Microcellular Communication System", U.S. Pat. No. 5,437,055, July 1995; The most aggressive schemes, often referred to as Spatial-Division Multiple Access (SDMA), allow multiple terminals to share one conventional channel (frequency, time) through different spatial channels, thereby multiplying the system capacity without additional frequency allocation (S. Andersson et al., "An Adaptive Array for Mobile Communication Systems," IEEE Trans. on Veh. Tec., Vol. 4, No. 1, pp 230-236, 1991; R. Roy et al., "Spatial Division Multiple Access Wireless Communication Systems", U.S. Pat. No. 5,515,378, April 1996, U.S. Cl.).
The key operations in SDMA involve spatial parameter estimation, spatial multiplexing for downlink (from the base station to remote terminals) and demultiplexing for uplink (from remote terminals to the base station). Since most of the current wireless systems adopt Frequency-Division-Duplex (FDD) schemes, i.e., different carriers for uplink and downlink (e.g., AMPS, IS-54, GSM, etc.), basic physical principles determine that the uplink and downlink spatial characteristics may differ substantially. Consequently, spatial operations in most SDMA schemes rely on direction-of-arrival (DOA) information of the terminals. More specifically, spatial multiplexing/demultiplexing is performed by separating co-channel signals at different directions.
While theoretically sound, there are critical practical problems with the current SDMA technologies, the most important ones being (i) computationally demanding algorithms for DOA and other spatial parameter estimation; (ii) a stringent requirement for calibrated system hardware;(iii) performance susceptible to motions and hardware/software imperfections. The first problem may be alleviated in a time-division-duplex (TDD) system (e.g., CT-2 and DECT) where uplink and downlink have the same propagation patterns. In this case, a terminal's spatial signature, i.e., the antenna array response to signals transmitted from the terminal, can be utilized in SDMA--no individual multipath parameters is required. Nevertheless other key problems remain. These problems may vitiate the usefulness of SDMA in wireless, and especially mobile communication networks.
It is worth pointing out that the above problems are not inherent to antenna arrays, rather, they are due to the rigid exploitation of the spatial diversity in order to accommodate the existing wireless protocols. The spatial diversity, which are highly unstable in nature, cannot provide reliable channels for communications. Any attempt to add smart antennas to existing systems can only leads to sub-optimum results. From a system viewpoint, there is an evident need for a specially designed scheme which utilizes start-of-the-art wireless technologies including the smart antennas in a unified fashion. The present invention meets this requirement and provides solutions for all the aforementioned difficulties.